Antimony trioxide and other powdered, solid flame retardants and flame retardant synergists have presented problems in handling and particularly in compounding the powdered flame retardants into polymers. A finer particle size solid polymer additive leads to better dispersion in a polymer matrix, and better dispersion equates to more efficient performance and improved polymer physical properties. Therefore, the solid polymer additives often have particle sizes reduced to less than 10 microns. Finely powdered polymer additives, however, present particular problems.
One problem of finely powdered polymer additives is dusting. The creation of dust involves loss of raw material, increased clean-up costs, and health concerns for those handling the solids. Another problem is bulk density. Finer solids tend to have decreased bulk density and increased packaging size, volume and cost.
The fluffy nature and low bulk density of finely powdered, polymer additive solids adversely affects additive flow properties, making them less compatible with solids which do not contain appreciable volumes of air. For example, low bulk density powders do not mix well with plastic pellets, resulting in poor homogeneity of the processed plastic containing these powders. More specifically, poor solids mixing homogeneity results in poor performance in flammability tests and poor physical properties in the final product.
A number of measures have been taken to overcome the problems of low bulk density fine powders. They all involve adding extra steps to the process of incorporating solid flame retardants and flame retardant synergists into polymers for the purpose of rendering the polymers less combustible.
One prior art approach to increasing the mixing homogeneity in the addition of low bulk density solids to polymers involves adding a liquid, such as a plasticizer, to the powder, prior to mixing the powder with the polymer. Another prior art approach involves blending a meltable carrier with the powder by means of heat and/or friction. Both approaches involve an additional step and incorporate an undesirable extraneous substance into the fmal polymer.
Blending the powder into the polymer in the form of a concentrate that can be diluted with more polymer to achieve the desired fmal concentration of powder additive is a common approach. It decreases dusting during the ultimate polymer processing step. However, it not only adds a costly additional step, but it also does not deal with the problem of poor mixing of a low bulk density powder and a polymer in forming the concentrate. In fact, the concentrate may have poorer homogeneity because a higher proportion of incompatible fine powder is added.
The approach described in U.S. Pat. No. 4,849,134 to solving these problems is cold compaction of the flame retardant and/or flame retardant synergist. The disadvantage of this method is that compaction (re-)agglomerates the fine particles. Unless subsequent polymer processing conditions result in complete breakup of coarse compacted (agglomerated) particles and dispersion into the polymer, the advantages of the fine particles are lost.
The process of vacuum de-aeration is known for other purposes, such as in processing cosmetics, food items, and thermoplastic resins, for example. Suitable process and equipment descriptions are found in U.S. Pat. Nos. 4,904,285 and 5,531,252, hereby incorporated by reference. However, vacuum de-aeration has not been used to increase the bulk density of flame retardants, flame retardant synergists, or other finely powdered polymer additives. Further, the prior art neither discloses nor suggests that increasing the bulk density of a finely powdered polymer additive improves processing speed of the polymer, better dispersion of the additive, and improved physical properties of the processed polymer.
Reducing the bulk density of flame retardants, flame retardant synergists, blends thereof, and other powdered polymer additives has significant value. These additives are included in an amount of about 1% to about 20%, often 10-20% by weight, of a finished polymeric article. Certain advantages of a lower bulk density polymer additive upon processing of one polymer, PVC, are referred to in U.S. Pat. No. 3,567,669. This patent discloses a high speed mixing process which requires a temperature of at least 170.degree. C. Under these conditions, the PVC particles have a "slightly sintered or glazed surface". Solid additives are absorbed or adsorbed onto the polymer surface.
U.S. Pat. No. 3,663,674 discloses densification of poly .alpha.-olefins. Cited advantages of increased bulk density are improved handling characteristics and the lack of a thermal history prior to processing.
No mention is made of the effect of the bulk density of powdered additives upon the processability or properties of the polymer. Nor is there any mention of the use of flame retardants or flame retardant synergists. Based upon the teachings of the '669 and '674 patents, one would not expect that increasing the bulk density of powders, such as fillers or pigments, would enhance the processability, achieve better dispersion of the additive throughout the polymer, or enhance the properties of the processed polymer.
A novel, improved solution to these problems has been found through the use of high bulk density, low dusting solid flame retardants, flame retardant synergists, blends, and other compacted, powdered polymer additives which maintain their fme particle size and improve the efficiency of the compounding step, improve the dispersion of additive more homogeneously throughout the polymer, and improve one or more properties of the compounded polymer.